Field of the Invention
The present invention relates to a method (process) for producing a foamed molded product (foamed molded object, or foam molded product) and an apparatus (device) producing the same.
Description of the Related Art
In recent years, injection foam molding methods using supercritical nitrogen or carbon dioxide as a physical foaming agent have been studied and practically used (Patent Literature 1: Japanese Patent No. 2625576 corresponding to U.S. Pat. No. 5,158,986; Patent Literature 2: Japanese Patent No. 3788750; and Patent Literature 3: Japanese Patent No. 4144916 corresponding to U.S. Pat. No. 5,997,781). According to each of Patent Literatures 1 to 3, the injection foam molding method using the physical foaming agent is performed as follows. First, the physical foaming agent is introduced into a hermetically closed plasticizing cylinder, and the physical foaming agent is brought in contact with and dispersed in a plasticized and melted resin. The molten resin, in which the foaming agent is dispersed, is measured while maintaining high pressure in the plasticizing cylinder to such an extent that the physical foaming agent is in a supercritical state, and then the molten resin is injected and charged into a mold. The supercritical fluid, which has been compatibly dissolved in the molten resin, is subjected to sudden pressure reduction and gasified at the time of injection and charging. The molten resin is solidified, and thus foams (foamed cells) are formed at the inside of the molded product.
Patent Literature 4 (Japanese Patent Application Laid-open No. 2013-107402 corresponding to U.S. Patent Application Publication No. 2013/0285273) discloses an injection foam molding method using a physical foaming agent, wherein the physical foaming agent in a molten resin is partially separated from the molten resin in the middle of molding, and the separated physical foaming agent is discharged to the outside of a plasticizing cylinder (kneading apparatus). Patent Literature 4 discloses the kneading apparatus including a vent from which the physical foaming agent is discharged and a mechanism that keeps the pressure of an area having the vent (pressure reduction zone) constant.
Meanwhile, a method for producing a foamed molded product using a chemical foaming agent in place of the physical foaming agent is suggested. In the method, a foaming agent that is decomposed by heat generated when a thermoplastic resin is plasticized and melted, is added to the thermoplastic resin, and foams (foamed cells) are formed at the inside of the molded product by gas generated when the foaming agent is decomposed. The molding method using the chemical forming agent eliminates an apparatus supplying the physical foaming agent, thus making it possible to reduce apparatus costs.
Patent Literature 5 (Japanese Patent Application Laid-open No. 2002-264164) discloses a method of producing a foamed molded product using a physical foaming agent and a chemical foaming agent in combination. According to Patent Literature 5, the amount of chemical foaming agent used can be reduced and foamed cells can be fine or minute by mixing, at a fixed ratio, a thermoplastic resin with hydrogen carbonate and carboxylate as chemical foaming agents and using a physical foaming agent with high pressure.
In the injection foam molding method using the physical foaming agent described in each of Patent Literatures 1 to 3, when concentration of the physical foaming agent in the molten resin is high, a phase separation between the molten resin and the physical foaming agent might occur. Thus, the concentration of the physical foaming agent needs to be decreased to about ⅕ to 1/10 of saturated solubility. In order to form many foaming nuclei during injection and charging into the mold, the physical foaming agent introduced into the plasticizing cylinder needs to be set to have high pressure and the introduced amount thereof needs to be measured precisely in a state where the concentration of the physical foaming agent in the molten resin is set to be lower than the saturated solubility. Those needs complicate a supply apparatus for the physical foaming agent, thus increasing apparatus initial costs.
In the injection foam molding method using the physical foaming agent described in Patent Literature 4, the kneading apparatus can increase concentration of the physical foaming agent in the molten resin almost to the saturated solubility (saturated concentration) after discharging a part of the physical foaming agent. This makes it possible to form many foaming nuclei by using the physical foaming agent with relatively low pressure.
The injection foam molding method described in Patent Literature 4, however, has the following problems. Inert gases, such as carbon dioxide and nitrogen, used as the physical foaming agent are not likely to dissolve in resin. When the thermoplastic resin having low compatibility with inert gases is used in a state where pressure in the pressure reduction zone is low, the foaming nuclei are not formed at the inside of the molded product sufficiently, which could cause increase in cell size and decrease in cell density. Meanwhile, when the pressure in the pressure reduction zone is high, the foamed cell size is reduced and the cell density is increased. In that case, however, there is a problem that separation between the molten resin and the physical foaming agent easily occurs and any unsatisfactory appearance, such as a swirl mark and swell, is likely to be present on a surface of the molded product.
The foam molding method using the chemical foaming agent has the following problem. In the foam molding method using the chemical foaming agent, when a small amount of the foaming agent is used, foamed cells are not formed sufficiently. Thus, for example, the chemical foaming agent is required to be added to the thermoplastic resin in a large amount exceeding 2% by weight. The chemical foaming agent is expensive, thus increasing material costs of the molded product. Further, dirt caused by decomposition residues from the chemical foaming agent is likely to adhere to the molding machine and mold, causing molding failure.
In the method using the chemical foaming agent and physical foaming agent in combination described in Patent Literature 5, the physical foaming agent with high pressure needs to be controlled such that it is introduced into the plasticizing cylinder at a specific timing in one cycle of the molding step. This requires a complicated control apparatus and an introducing mechanism for introducing the physical foaming agent, thus increasing apparatus costs.
The present teaching is made to solve the various problems related to the methods of producing the foamed molded product. According to the present teaching, there is provided a method of producing a foamed molded product using a physical foaming agent and a chemical foaming agent in combination, wherein the amount of chemical foaming agent used may be reduced, and a complicated control apparatus for a fluid functioning as the physical foaming agent may be omitted or simplified.